“With my characteristic optimism, my 1997 paper on global warming remarked that ‘We should know a lot more about this issue in the next 10 to 20 years.’ Unfortunately, there has been a rush to judgement on this issue without a significant increase in the information on which to base this judgement.”
– William Niskanen, 2008
Part I yesterday presented the key questions regarding the climate-change issue from William Niskanen’s Fall 1997 symposium essay, “Too Much, Too Soon: Is a Global Warming Treaty a Rush to Judgment?” Part II today reprints a lead section from that essay, How Good is the Science of Global Warming? followed by Niskanen’s eleven-year retrospective. I conclude with a brief comment as a twenty-one year retrospective.
The current debate in the scientific community about global warming is based on only a few hard facts: The current concentration of carbon dioxide in the atmosphere is nearly 30 percent higher than in pre-industrial times; the average (measured) global temperature has increased about 0.5°C since the 1880s; and the increased concentration of carbon dioxide may have contributed to the increase in temperature. One might hope for a more solid scientific foundation for a major international treaty. According to a May 16, 1997, article in Science magazine, however,
… many climate experts caution that it is not at all clear that human activities have begun to warm the planet—or how bad greenhouse warming will be when it arrives.
The substantial remaining scientific issues about global warming will have to be sorted out within the scientific community, preferably by scientific standards rather than by a politically forged consensus. Sooner or later, however, scientists should be asked to explain some conditions that puzzle informed and intelligent people in the broader community: The increase of 0.5°C in the average (measured) global temperature seems very small relative to the variance of natural conditions and possible measurement errors.
Natural water vapor constitutes about 98 percent of global warming gases, so a small variation in water vapor or the reflectivity of the atmosphere would overwhelm the effect of a change in carbon dioxide concentration. For this economist, it also seems implausible that average global temperatures were measured very accurately over the past century.
The relative timing of the increase in average global temperature and carbon dioxide emissions also seems inconsistent with an anthropogenic interpretation of the temperature increase. Most of the temperature increase occurred by 1940, but most of the carbon dioxide emissions occurred in the subsequent years.
Although the 1980s was an unusually warm decade, neither the satellite nor the weather balloon measures of the temperature in the lower atmosphere show any net warming over the past 18 years. Scientists may yet demonstrate that a continued increase in carbon dioxide concentration will increase the average temperature, but such evidence is not yet sufficient to support measures that would impose substantial economic costs.
Science Postscript (2008)
With my characteristic optimism, my 1997 paper on global warming remarked that “We should know a lot more about this issue in the next 10 to 20 years.” Unfortunately, there has been a rush to judgement on this issue without a significant increase in the information on which to base this judgement.
The cumulative increase in average world temperatures since the 1880s is now estimated to be 0.80C, but there is no clear relation between the level of carbon dioxide in the atmosphere and the average level of global temperatures. Much of the estimate increase in temperature was before 1940, prior to any substantial accumulation of carbon dioxide in the atmosphere.
There is still no explanation of the significant cooling from 1940 until 1975, despite a substantial increase in atmospheric carbon dioxide; the climate alarmists of the 1970s were predicting a new ice age. The historical record does not yet provide sufficient evidence that the increase in atmospheric carbon dioxide is the primary cause of the increase in average world temperatures.
The global warming “pause” or “hiatus” since Niskanen’s (2008) postscript adds to the debate about a strict causality between CO2 levels and both temperature and sea-level rise. A new term, global lukewarming, has emerged that Niskanen would have surely embraced as the middle ground on the science side.
Niskanen would have also noted that a decade later, we still are not on firm scientific ground, although the evidence continues to move away from high-sensitivity (warming) from the enhanced greenhouse effect.
In fact, just in time for this post, Judith Curry updates the Great Climate Debate with her usual candor and facility for summary in “The Debate: My Presentation.”
I’ve been asked to respond to the question “To what extent are man-made CO2 emissions contributing to climate change?”
The short answer is: ‘we don’t know.’ The reason is that we don’t know how to disentangle natural internal variability from the effects of CO2–driven warming
Even the IPCC doesn’t claim to know exactly. The most recent IPCC assessment report says it is ‘extremely likely’ to be ‘more than half.’ ‘More than half’ is not very precise.
Given the IPCC’s neglect of multi-decadal and longer time scales of natural internal variability, I regard the extreme confidence of their conclusion to be unjustified.
So here is my personal assessment, using the jargon of the IPCC: Man-made CO2emissions are as likely as not to contribute less than 50% of the recent warming.
Bill Niskanen would nod his head in agreement.